These are the Supplementary Files for the article "The fate of plasma-generated oxygen atoms in aqueous solutions: non-equilibrium atmospheric pressure plasmas as an efficient source of atomic O<sub>(aq)</sub>" with abstract:<div><br></div><div>Non-equilibrium radio-frequency driven atmospheric-pressure plasma in He/0.6%O<sub>2</sub> gas mixture has been used to study the reaction mechanism of plasma-generated oxygen atoms in aqueous solutions. The effluent from the plasma source operated with standard and <sup>18</sup>O-labeled O<sub>2</sub> gas was used to treat water in the presence of phenol as a chemical probe. Comparing the mass spectrometry and gas chromatography-mass spectrometry data of the solutions treated with plasma under normal and labeled oxygen provides clear evidence that O<sub>(aq)</sub> originating from the gas phase enters the liquid and reacts directly with phenol, without any intermediate reactions. Additionally, the atmospheric-pressure plasma source demonstrates great potential to be an effective source of O<sub>(aq)</sub> atoms without the requirement for any precursors in the liquid phase.</div>